Survivors and Zombies: How the Milky Way Built Its Satellite Family

Debosmita Pathak and collaborators explore what happens to the tiny galaxies that fall into Milky Way–like systems. Some survive to the present day as faint satellites, while others are destroyed and leave behind streams of stars. By using detailed computer simulations, the authors compare both types, “survivors” and “zombies”, to understand how galaxies like the Milky Way grew over time.

Introduction: Building the Milky Way

Astronomers know that the Milky Way has grown through mergers, big galaxies eating smaller ones. Evidence comes from two places: (1) the stellar halo, which contains debris from destroyed galaxies, and (2) the surviving dwarf satellites we see today. However, the satellites we see now are only the tip of the iceberg, most galaxies that fell into the Milky Way have already been disrupted. Pathak and collaborators argue that comparing the disrupted and surviving populations is key to understanding why some satellites quench their star formation and others get destroyed.

Simulations and Methods

To investigate, the team used the D.C. JUSTICE LEAGUE suite of high-resolution simulations, which track the evolution of Milky Way–like galaxies over 12 billion years. These simulations follow both dark matter and ordinary matter (gas and stars), allowing them to model how satellites form stars, lose gas, and eventually survive or disrupt. The authors defined satellites based on whether they ever entered the main galaxy’s halo and then divided them into four categories: star-forming survivors, quenched survivors (those that stopped forming stars), independently quenched satellites, and disrupted satellites.

Results: Metallicity and Star Formation

One of the clearest differences between survivors and disrupted galaxies comes from their chemical makeup. Disrupted galaxies tend to be poorer in iron ([Fe/H]) but richer in “alpha” elements like oxygen ([O/Fe]). This points to faster, earlier star formation compared to surviving dwarfs. The simulations reproduced observed trends: disrupted galaxies show signs of forming stars quickly and then being destroyed, while surviving dwarfs often had slower, longer star-formation histories. For ultra-faint dwarfs, the tiniest systems, the simulations showed they typically quenched before falling into the Milky Way, likely because of reionization early in the universe.

Results: Mass, Infall Time, and Survival

Whether a satellite survives depends heavily on both its mass and the time it entered the Milky Way. Satellites that fell in more than 10 billion years ago almost never survive to the present, while those that arrived more recently are much more likely to still be around. High-mass satellites are also more prone to disruption because their large size makes them lose orbital energy faster and sink toward the Milky Way’s center. The simulations showed that nearly all disrupted satellites kept forming stars right up until they were destroyed, while survivors often quenched earlier.

Results: Orbits and Disruption

Orbit shape also matters. Satellites on radial orbits, those that dive in close to the Milky Way, are more likely to be destroyed or quenched quickly, compared to satellites on wide, circular orbits. The timescales are fast: low-mass satellites can be disrupted in less than a billion years, which is nearly the blink of an eye in cosmic terms.

Discussion and Implications

By comparing quenching and disruption, the authors find that massive galaxies usually disrupt before quenching, while small ultra-faint dwarfs quench long before disruption. This difference explains why the Milky Way’s halo is filled with the remnants of massive but short-lived galaxies, while the tiniest galaxies often survive. The study also strengthens the connection between today’s faint dwarfs and the long-gone galaxies that once helped build the Milky Way’s halo.

Conclusion

Pathak and colleagues demonstrate that the survival or destruction of a satellite depends on a delicate interplay of its mass, when it fell in, and its orbit. Disrupted galaxies were often still forming stars until the very end, while ultrafaint dwarfs quenched long ago but managed to persist. This work helps bridge the gap between the Milky Way’s visible satellites and the ghosts of galaxies past, offering a more complete picture of how our Galaxy assembled.

Source: Pathak